Here's how SPF works:
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Similarly to DKIM, SPF verifies that emails are really coming from you. Your 'SPF record' is essentially a note in your DNS that Act-On has permission to send messages on your behalf. If Act-On is not specified in this SPF entry, most email servers will assume that we do not have permission to send your emails. This means that your messages will automatically be considered questionable and may not be delivered to your recipients.
To set up SPF, please see Required Technical Setup.